This invention relates to a fluorescent display panel having an indirectly heated cathode, and more particulary, to the structure of the indirectly heated cathode.
One prior art fluorescent display panel uses an insulated substrate with displays and anodes mounted thereon. This structure is sealed in a vacuum glass envelope. The display is viewed through the envelope.
In contrast, another fluorescent display panel is disclosed in U.S. Pat. No. 4,455,774. An insulative substrate is made transparent, and an anode segment group is disposed on the insulating substrate. The segment group is made of a transparent material or is formed with a mesh structure. A fluorescent material is coated on the anode segment. The fluorescent display is observed through a surface of the substrate which is opposite to the surface on which the anode segment is formed.
In both of these types of fluorescent display panels, the cathode wire is stretched in common across all of the indicating patterns. Accordingly, a different potential distribution is created, with respect to the voltage of an operating circuit, within the voltage applied to the cathode wire. This potential distribution appears between the cathode wire and each of the anode segments constituting the display pattern. As a result, when the cathode wire is heated and actuated by a DC power source, a fluorescent display panel is partially illuminated, in different degrees of brightness. Particularly, when there is a multi-digit display panel having a long cathode wire to which a higher heating voltage is applied, the display patterns are illuminated in different brightness which vary one pattern after another.